1. Field of the Invention
The invention relates to an X-ray source for generating substantially monochromatic fluorescent X-rays with a primary and a secondary target.
2. Description of the Related Art
An X-ray source of this kind is known from U.S. Pat. No. 3,867,637 and includes, accommodated in an X-ray tube, essentially a (primary) target which faces a cathode and in which X-rays are produced by the incidence of an electron beam. The target bears on a substrate which may be made, for example of a light metal such as aluminum or beryllium and serves for mechanical support of the target and for ensuring vacuumtight sealing of the X-ray tube. The substrate is essentially transparent to the X-rays emanating from the target and is chosen to be so thick that all incident electrons are absorbed. On the other side of the substrate there is provided a fluorescent material (secondary target) which may be, for example cerium oxide, so that the X-rays that are incident from the primary target excite material-dependent monochromatic fluorescent X-rays.
A problem encountered in such known X-ray sources consists in that it is comparatively difficult to couple a large part of the X-rays generated in the primary target into the secondary target. Consequently, the intensity of the excited monochromatic fluorescent X-rays is also low or can be increased only by modification of the target at the expense of the spectral purity.
Therefore, it is an object of the invention to provide an X-ray source of the kind set forth whereby essentially monochromatic fluorescent X-rays can be generated with a high radiation intensity and at the same time a high spectral purity.
This object is achieved by means of an X-ray source of the kind set forth which is characterized in that the primary target is a liquid metal or a liquid metal alloy which is conducted between a first window, being transparent to an electron beam, and a second window, being transparent to X-rays and adjoined by the secondary target, in such a manner that electrons which are incident on the primary target via the first window produce X-rays which exhibit, upon reaching the secondary target, essentially a maximum energy which corresponds to an absorption edge of the secondary target so that substantially monochromatic fluorescent X-rays are excited in the secondary target.
The (at least in the operating condition of the X-ray source) liquid metal or the metal alloy serves not only as a primary target, but at the same time provides effective dissipation of heat from the target and also cools the windows; a comparatively strong development of heat occurs notably at the first window due to the incident electron beam. As a result of the cooling, the electron incidence and hence the thermal power density can be significantly increased, so that the radiation intensity of the monochromatic fluorescent X-rays is increased accordingly.
The dependent claims disclose advantageous further embodiments of the invention. The embodiment of the windows as disclosed in claim 2 offers the advantage that on the one hand these windows are particularly stable so that they are capable of withstanding the streaming pressure of the flowing liquid metal even when they have a comparatively small thickness whereas on the other hand they extract only a very small amount of energy from the electron beam or the X-ray beam.
The embodiment disclosed in claim 3 offers the advantage that particularly effective dissipation of heat from the windows is achieved.
Finally, the embodiment disclosed in the claims 4 and 5 offers a substantial enhancement of the spectral purity of the X-rays coupled out from the secondary target.